Semiconductor memories may be divided into two classes, volatile and nonvolatile. In volatile memories the stored data is lost when power is removed, and in non-volatile memories the stored data is not lost when power is lost. Because of the need for static non-volatile memory in calculators, watches, game products, and other commercial devices, there has been a increasing demand for these type of memories. Typical non-volatile memories include Read-Only Memories (ROMs), Erasable Programmable Read-Only Memories (EPROMs), Flash EPROMs, and Electrically Erasable Programmable Read-Only Memories (EEPROMs). In a ROM, the contents are defined at the time of fabrication by selective patterning of one or more fabricated layers. However, a ROM cannot be rewritten if the user wants to modify the data stored in the ROM. Rather the user must re-specify the desired memory contents and the revised chips must be fabricated with new information stored. Erasable Programmable Read Only Memories (EPROM) are erased by exposing the memory array to ultraviolet light which allow the charge stored on a Floating Gate Avalanche Injection Metal Oxide Semiconductor (FAMOS) device floating gate to discharge to the surrounding conductive materials, thereby erasing the state of the cell. To rewrite the memory, an elevated voltage supply is required to tunnel charge to the desired FAMOS floating gates. The drawbacks of EPROMs include long erase times (.about.20 minutes), expensive windowed packaging, process complications due to the high programming voltage required, and the fact that EPROMS must be removed from the system to be re-configured.
Electrically Erasable Programmable Read Only Memories (EEPROM) are electrically erasable by the application of an elevated voltage which overwrites the charge on the floating gate of the FAMOS device. This has obvious advantages over the EEPROM technology in that devices can be quickly erased while still in the system and can be purchased in inexpensive plastic packages. EEPROMs have the disadvantage of requiring the system designer to route the elevated programming voltage as well as the normal supply voltage to their design. The erasing is typically done in large blocks, so while prototyping the user must completely reload the memory contents when any change is made.
Flash technology is the latest successful entrant into the non-volatile memory business. Flash memories are similar to EEPROMs except that the erasing voltage is the same as the normal power supply. Again, the erasing is typically done in large blocks, so while prototyping the user must completely reload the memory contents when any change is made.